This invention relates to an electrochemical battery cell with a nonaqueous organic solvent electrolyte and an improved thermoplastic sealing member.
Nonaqueous battery cells are cells that contain essentially no water. The cell electrode materials and electrolyte are carefully manufactured, dried and stored prior to cell manufacturing to maintain the amount of water in those components at typically no more than tens or hundreds of parts per million. Those manufacturing processes in which cell internal components are exposed to the air are generally performed in a dry box or a dry room. These measures are necessary because of the high reactivity of one or more of the cell ingredients with water. Organic compounds are often used as electrolyte solvents in nonaqueous cells. Examples of nonaqueous cells that contain such organic solvents include lithium and lithium ion cells, though other types of nonaqueous cells, containing other materials that are highly reactive with water are known.
Batteries containing nonaqueous cells are becoming increasingly popular as power sources for electronic devices. Though they are often more costly than common aqueous cells, nonaqueous cells can have many advantages because of the natures of materials used. These advantages include high energy density, high capacity at low temperatures, low weight and excellent shelf life over a broad range of temperatures. Many nonaqueous cells also have high electrode interfacial surface area designs that make them especially well suited for high power (including high current and low resistance) discharge, and the general trend in power requirements for electronic devices has been toward higher and higher power. Some of the types of devices for which high capacity on high power discharge is particularly important include photoflash devices (flash units and cameras with internal flash capability), digital still cameras, video cameras, personal digital assistant devices and portable computers.
The ability to withstand extreme temperature conditions, including thermal cycling and thermal shock between high and low temperatures, is becoming more important for nonaqueous cells, particularly lithium and lithium ion cells larger than button cells.
Requirements for lithium and lithium ion cells to tolerate extreme temperature conditions without seal degradation resulting in salting, leakage, excessive weight (electrolyte) loss, venting at low internal cell pressures and excessive capacity loss are increasing. This is true from the standpoint of both the severity of the conditions that the cells must tolerate and the number and types of applications for which such requirements are being set. Cells with thermoplastic seal members made according to the prior art are not able to meet all of these requirements in certain cell types, particularly cells with electrolyte solvents having low boiling points.
A wide variety of cell designs have been used for nonaqueous cells. The type of design is dependent in part on the size of the cell, the type of electrode and electrolyte materials used in the cell and the power requirements of the devices to be powered by the cell. Because the cathode/electrolyte materials are so reactive, the designs for large liquid cathode lithium cells (e.g., lithium-sulfur dioxide (Li/SO2) and lithium-thionyl chloride (Li/SOCl2)) often have housings in which metal components are hermetically welded, and glass seals are used to seal metal components that must be electrically insulated and to seal small apertures in the housings. These types of housings tend to be expensive due to the materials and the manufacturing processes and equipment required.
Other means can be used to seal the cells. Because of the relatively low cost and ease of manufacture, it can be desirable to use thermoplastic seal members between rigid housing components. For example, a thermoplastic gasket or grommet can be compressed between the inside top edge of the cell container (e.g., a steel can) and the periphery of the cover closing the open top of the can, forming a seal to keep the electrolyte within the cell housing and to keep water out.
A thermoplastic seal member can also be used to seal an aperture in the cell housing. For example, the thermoplastic seal member may be in the form of a plug sealing a small hole in the cell cover. Electrolyte may be dispensed into the cell after the cover has been assembled to the can. In another example, the plug may be a rigid material, such as a glass or metal ball, with a thermoplastic seal member in the form of a bushing between the inner surface of the aperture and the ball. In these examples, the thermoplastic plug or the ball and bushing may also function as a pressure relief vent for the cell.
FIG. 1 shows an example of a cylindrical lithium cell design that has been used for Li/FeS2 and other lithium cell types. It has two thermoplastic seal members—a gasket sealing a cover in the open end of the can and a bushing sealing an aperture in the cell cover. Both thermoplastic seal members provide a compressive seal. Since the can and cover are electrically connected to opposite electrodes within the cell, the gasket also provides the necessary electrical insulation. The bushing and a vent ball comprise a pressure relief vent for the cell. When the internal cell pressure exceeds a predetermined abnormally high level, the vent ball (or the ball and bushing) are forced out of the cover, leaving an opening through which pressure is released. Cells sealed with both a gasket between the can and cover and a pressure relief vent comprising a bushing and vent plug disposed in an aperture in the cell cover are disclosed in U.S. Pat. Nos. 4,329,405 (issued May 11, 1982), 4,437,231 (issued Mar. 20, 1984), 4,529,673 (issued Jul. 16, 1985), 4,592,970 (issued Jun. 3, 1986), 4,927,720 (issued May 22, 1990) and 4,931,368 (issued Jun. 5, 1990) and 5,015,542 (issued May 14, 1991), the entire disclosures of which are incorporated herein.
Thermoplastic seal members are also used in other types of cells, including aqueous electrolyte cells such as common consumer type aqueous zinc-manganese dioxide (Zn/MnO2), nickel-cadmium (Ni/Cd) and nickel-metal hydride (NiMH) cells.
For any cell type, the seal member material and design must be such that a suitable seal is maintained for an acceptable period of time and under the temperature conditions that the cell is expected to withstand during transportation, storage and use. Common characteristics of a good seal member include stability of the material in the internal cell and external environments, impermeability to the liquids and gases that are to be sealed within or outside the cell, and the formation and maintenance of a complete seal path (i.e., with no voids or gaps) at each seal interface.
For thermoplastic seal members which form a compressive seal, the seal member must be sufficiently compressed to achieve a good seal, and sufficient compression must be maintained for the desired time. Thermoplastic materials under compressive stress tend to move to relieve that stress. This is referred to as stress relaxation or cold flow of the material. Thermoplastic materials tend to stress relax more at higher temperatures, thereby reducing the time that sufficient compression can be maintained. Temperature also affects the compression of thermoplastic seal members in another way. Different materials will expand and contract by different amounts in response to increases and decreases, respectively, in ambient temperature. In a cell with a thermoplastic seal member forming a compressive seal between more rigid components (e.g., a metal can and a metal cover), it is generally desirable for the gasket and rigid components being sealed to expand at close to the same rate in order to maintain sufficient gasket compression over the greatest temperature range possible.
Thermoplastic materials and seal designs suitable for nonaqueous cell seal members are more limited than for aqueous cell seal members. Because active materials in the cell are very reactive with water, the seal members must have a higher degree of impermeability to water, and some common materials for aqueous cell seal members are not suitable. Nonaqueous cell seal members must also have a low vapor transmission rate for the electrolyte solvents. Since the vapor transmission rate of thermoplastic material is generally dependent in part upon the vapor pressure of the solvent, low vapor transmission rates are generally more difficult to achieve for nonaqueous cells whose electrolytes contain ethers or other organic solvents with low boiling points. The greater the ratio of the effective cross sectional area of the seal member to the internal volume of the cell, the more important the electrolyte solvent and water transmission rates.
For use in some devices, such as those that may be used in automobile engine compartments and some outdoor environments, batteries must be capable of withstanding very high or very low temperatures. Electrochemical characteristics of some lithium and lithium ion cells make them desirable for use at such temperature extremes. However, seal members used in cells intended for such applications must be able to maintain an acceptable seal at those extreme temperatures. The importance of resistance to the effects of temperature extremes is becoming more important.
Polypropylene (PP) is commonly used a material for lithium cell (e.g., Li/MnO2 and Li/FeS2) gaskets. Gaskets have been made with other thermoplastic materials for the purpose of improving the ability of the cell to withstand high temperatures than with PP.
Sano et al. (U.S. Pat. No. 5,624,771) disclose the use of polyphenylene sulfide (PPS), rather than PP, as a gasket material for a lithium cell to improve resistance of the cell to high temperatures. PPS was used to reduce gasket deformation due to cold flow under the high load conditions the gasket was subjected to in the cell.
In U.S. Pat. No. 5,656,392, Sano et al. disclose thermoplastic synthetic resins, PPS and tetrafluoride-perfluoroalkyl vinylether copolymer (PFA), suitable for making a gasket for a cell that is useable at high temperatures and solves conventional problems caused by long-period use and/or storage. Also disclosed are the addition of a glass fiber filler to the resin to extend the stability of the gasket configuration and the addition of small amounts of polyethylene (PE) and/or polypropylene (PP) to extend the temperature range that can be tolerated by the cells on a cyclic thermal shock test. However, gaskets containing more than 10 weight percent glass fiber were undesirable because cells made with such highly filled thermoplastic materials leaked on a temperature cycling test. The addition of more than 10 weight percent of PE and/or PP was also undesirable because of cell leakage and a continuously usable temperature of less than 150° C. for the gasket.
Both U.S. Pat. Nos. 5,624,771 and 5,656,392 teach that high boiling point solvents such as γ-butyrolactone (boiling point 202° C.) and propylene carbonate (boiling point 241° C.) can be used as electrolyte solvents to achieve the desired high temperature cell performance and still maintain practical low temperature (−20° C.) cell operation in a Li/(CF)n coin cell. However, lithium cells with electrolytes containing a large amount of low boiling point solvents do not perform as well on high power discharge, which can be a disadvantage in larger cells intended for use in high power discharge applications.
In U.S. Pat. No. 6,025,091 Kondo et al. disclose a cell with a metal can sealed with a metal terminal cap and a gasket comprising polybutylene terephthalate (PBT). The gasket material can be PBT alone, PBT mixed with another polymer or PBT reinforced with inorganic materials such as glass fibers, glass beads and certain organic compounds. Kondo et al. disclose that the invention solves the problems of creeping and cracking of the gasket material when the cell is exposed to high temperature. The preferred cell type was a secondary cell, either with an alkaline or nonaqueous electrolyte (e.g., a lithium ion cell). A particularly preferred electrolyte contained LiCF3SO3, LiClO4, LiBF4 and/or LiPF6 dissolved in a mixed solvent comprising propylene carbonate or ethylene carbonate and 1,2-dimethoxyethane and/or diethyl carbonate and 1,2-dimethoxyethane and/or diethyl carbonate.
In the mid-1980's Union Carbide Corp. also manufactured a ⅓ N size Li/MnO2 cell (Type No. 2L76) with a gasket made from PBT (GAFITE® from GAF Chemicals). These cells had a spiral wound electrode design and contained an electrolyte with comprising a mixture of lithium perchlorate and lithium trifluoromethanesulfonate salts in a solvent containing 50 volume percent each of propylene carbonate and 1,2-dimethoxyethane.
The prior art teaches that the ability of cells to withstand a wide range of temperatures, especially high temperatures, can be improved by using gaskets made from materials that maintain dimensional stability and do not crack under extreme temperature conditions. The problem of reducing the rate of transmission of electrolyte solvent through the gasket is not addressed. This problem is generally greater at higher temperatures and with more volatile organic solvents with lower boiling points, such as ethers.
When a pressure relief vent for the cell is incorporated into the seal member, the characteristics of the thermoplastic seal member that affect the operation of the pressure relief vent must also be considered when selecting a suitable thermoplastic resin. Ethylene-tetrafluoroethylene copolymer (ETFE) is commonly used for vent bushings in consumer Li/FeS2 cells with pressure relief vent designs similar to that in FIG. 1. When the internal cell pressure reaches a predetermined level, the vent ball or the vent ball and the vent bushing are forced outward to create an opening in the cell. When tested on a thermal shock test, the ETFE can sometimes undergo sufficient stress relaxation to cause a partial or complete loss of compression between the vent ball and cover or cause activation of the pressure relief vent undesirably low internal cell pressures.
Accordingly, battery cells with improved thermal tolerance characteristics, with little or no adverse effects on other cell characteristics, are desired. Therefore, an object of the present invention is to provide an economically made electrochemical battery cell, with a seal member made from one or more thermoplastic resins, having improved thermal tolerance characteristics. Another object of the invention is to provide a battery cell with a pressure relief vent comprising a thermoplastic seal member that has improved thermal tolerance characteristics.